This invention relates, in general, to electronic communication devices and, more specifically, to devices for detecting the presence of specialized electromagnetic signals.
Many aircraft and airborne weapon systems fly at relatively low altitudes and carry special instruments or devices for determining or measuring the distance between the flying object and the ground. Radio altimeters are frequently used for such purposes, with one of the most popular types using FM-CW (frequency modulated-continuous wave) techniques. In such altimeters, the transmitted wave from the airborne device consists of a continuous or non-pulsed signal modulated to shift the instantaneous frequency in predetermined amounts and directions.
In military applications, it is desirable to detect or know when an aircraft or missile is approaching. In many instances, this can be accomplished by conventional radar systems which use reflected signals from the airborne object produced by illuminating transmissions from a remote source. Although useful in many situations, conventional radar has certain limitations, especially with low flying aircraft.
Detection of airborne objects containing radio altimeters is sometimes more reliable or desirable than using conventional radar systems. Generally speaking, the detection is accomplished by monitoring for the presence of radio or electromagnetic signals from the altimeter device. Although successful altimeter detectors have been used, their accuracy and efficiency is not as good as desired in certain situations.
According to prior art arrangements, detection of altimeter signals is somewhat difficult due to the fact that the transmitter used by the altimeter has a relatively low power output. Values from one-half to one watt are typical. In addition, the transmitted energy reaching the detection device is leaving the altimeter antenna at a very low sidelobe of the antenna pattern. Therefore, little or no antenna gain is realized to increase the effective power received by the detecting device. Also, the propagation of the sidelobe signals in the direction of the detecting device is further attenuated by the aircraft fuselage along which the signal must travel to be directed ahead of the moving aircraft. Estimates of this sidelobe attenuation vary, but best information available places the attenuation such that antenna sidelobe gain is between −5 and −15 dbi. Another factor which makes detecting altimeter signals difficult is the fact that typical altimeters are modulated over several tens of megahertz. This makes it difficult to use conventional narrow band receiving and detecting techniques which offer better signal-to-noise detection capabilities.
For all of these reasons, detection has usually been limited to short ranges. Normal practice according to the prior art has been to use either narrow filters or compressive receivers for altimeter detection. The narrow filter arrangement senses only a small spectral portion of the transmitted energy and is rather limited in its ability to detect and, as is usually desirable, identify the source of altimeter signal as to, for example, friend or foe.. Therefore, it is desirable, and it is an object of this invention, to provide a reliable and efficient altimeter detector which affords long range detection capabilities along with the ability to look closely at the altimeter originated signal to determine the identification of the signal source.
Prior art considered pertinent to this invention is described in two U.S. patents. U.S. Pat. No. 3,296,581, issued on Jan. 3, 1967, teaches a detection technique whereby the input signal is applied to a coincidence detector both directly and through a delay line. The signal is first limited, differentiated, and clipped before being applied to the delay line which has a precise time period of one wave length. The coincidence detector produces an output only upon coincidence of its two input signals. As will be described later in more detail, the current invention does not require limiting, differentiating, or clipping the signals before application to the delay line, the delay of the delay line is much longer than one RF wave length and not fixed at that value, and the direct and delayed signals are applied to a mixer, or difference circuit, not to a coincidence detector.
U.S. Pat. 4,225,954, issued on Sep. 30, 1980, teaches a system which uses an auto correlation process to calculate delay times for various signals received by the system. The similarity of this patent with the present invention exist because both use fast Fourier transform processing, auto correlation, and analog-to-digital conversion to process an incoming signal. However, distinct differences exist which distinguish the present invention over this patent in addition to the fact that the patent pertains to audio signals whereas the present invention pertains to microwave frequency signals. In the referenced patent, the auto correlation is applied to a digital signal and produces digital values corresponding to time differences. The auto correlation process does not produce the signal to be processed by fast Fourier transform techniques as in the present invention, but produces variable quantities which are further processed with the results of the fast Fourier transform processing to produce a corrected frequency domain signal. As will be described in more detail herein, the present invention uses some of the same components as the referenced patent but for different reasons and at different steps in the processing technique.
Several prior art arrangements exist that teach the use of radar receivers which use delay lines in the signal processing system. The major difference between all of these prior art arrangements and the present invention is that the prior art arrangements use local oscillators which mix with the input signal to provide an IF signal. The present invention does not use a local oscillator but instead uses a delayed replica of the input signal to form the mixer injection frequency. The advantages of this technique are discussed in the detailed description of the present invention.